The theme throughout these three books is man's battle (or, rather, intelligent life's battle) between good and evil, with some very obvious, but not stifling, religious overtones also found in CS Lewis' nonfiction work. For adults who absolutely adored the Chronicles of Narnia set, this trilogy takes you through the battle between good and evil in a more sophisticated manner. Granted, these are not nearly as easy to read, but adapting to the more complex (sometimes slow-moving in Hideous Strength) writing style was quick.
If you are primarily interested in religious fiction, and have the patience to read books with more complexity than, say, the Left Behind series, you will like these allegorical journeys through the fall of man. If you are primarily interested in SciFi, CS Lewis takes you to other worlds (Silent Planet, Perelandra) and introduces beings from another Earth-time (Hideous Strength) with an original twist of the good vs. evil storyline.
All three books can be read on their own, however I found that "That Hideous Strength" would have been difficult to follow without the background provided in either "Out of the Silent Planet" or "Perelandra". Regardless of the individual readability of the 3 stories, I started with the 1st book (Out of the Silent Planet) not sure I would enjoy it, and ended up finishing all 3 within a week or two.